1. Field of the Invention
The present invention relates to a method for direct patenting of hot-rolled wire rod.
2. Description of the Prior Arts
Now at present, as a direct patenting method of a hot-rolled wire rod, Stelmor method is widely used. In this Stelmor method, a wire rod having been hot-rolled at a temperature of 800.degree. C. to 900.degree. C. are firstly coiled into a form of series of loops by a coiler, and the wire rod is dropped and introduced to a conveyer and is transported thereon in a state of being in a form of continuous series of loops. And then, the wire rod is forced to rapidly be cooled by air-blast at a rate of 10 m to 60 m/sec. from the down side of the conveyer during the transportation, thereby to strengthen the wire rod. The capability of the air blast cooling, however, is limited to a certain extent. When it comes to a wire rod with 11 mm in diameter, the speed of this air-blast cooling becomes so low as to be approximately at a rate of 5.degree. C. to 10.degree. C./sec. When a wire rod of high carbon steel is produced by this air-blast cooling, because of the low speed of the air blast cooling the wire rod is reduced to being low in its strength and ductility, compared with that which is produced in off line lead patenting (herein after referred to as "LP").
As one of the prior art means to cover this disadvantage, a method of using a warm water or salt bath is widely known. But, by means of this warm water, a speed of water cooling as much as that of LP cannot be obtained. On the other hand, the salt bath has disadvantages in that the salt bath not only takes much time to dissolve the salt in the salt bath but also increases investment cost for equipment and facilities as well as running cost.
As an alternative of said prior art means to more simply improve cooling capability of said Stelmor method, mist blasting method is disclosed in a Japanese Patent Application Laid Open (KOKAI) No. 112721/76, wherein water of 0.01 to 0.05 l/air-blast of 1.0 m.sup.3 is sprayed. Furthermore, in Japanese Patent Application Laid Open (KOKAI) No. 138917/78, it is disclosed that air-blast which is mixed with water of 0.06 to 0.27 l/Nm.sup.3 to turn into mist is used. These prior art methods, however, teaches only that cooling capability is improved by using merely mixing air-blast with water. According to LP, because of its use of a constant temperature bath at about 520.degree. C., pearlite transformation is performed at an isothermal transformation (TTT transformation), and the transformation is performed at the vicinity of a nose in a graphic representation of the transformation. Therefore, in LP, a fine pearlite structure can be obtained. But, when a continuous cooling transformation (CCT transformation) is carried out, in case of over cooling, bainite or martensite is produced even if the cooling capability is merely increased. Namely, even if rapid cooling is effected to the vicinity of the pearlite nose temperature, the pearlite transformation does not start yet, or just starts. Since thereafter most of austenite which is not transformed yet will begin to be transformed, the product quality becomes very fluctuant unless moreover temperature of finishing rapid cooling and heat treatment pattern subsequent to the rapid cooling are closely controlled. A wire rod which is produced by means of this continuous cooling method does not match, in quality, features of that produced by means of LP method.
Various methods have been studied to overcome said limit of the continuous cooling. Japanese Patent Application Laid Open (KOKAI) No. 41323/81 discloses a method wherein a wire rod which has been hot-rolled is cooled by means of control cooling at a cooling speed of forming a sorbite structure down to a temperature higher than a temperature Point M.sub.s at which martensite transformation starts, and subsequently the wire rod is reheated up to a temperature as high as TTT curve nose, thereby to procure a time enough for austenite not yet transformed to complete its transformation. Another method is also disclosed in Japanese Patent Application Laid Open (KOKAI) No. 214133/87, wherein after rapid cooling of a hot-rolled wire rod down to the vicinity of 550.degree. C. of the pearlite nose temperature, the temperature is constantly maintained by blasting a hot air at a temperature over M.sub.s point but less than the nose temperature. These methods of reheating a wire rod to a certain temperature to keep the temperature constant by reheating and of using hot air blast both are disadvantageous in that they require a great deal of construction cost, compared to a method of performing a slow cooling simply.
Recuperation occurring due to exothermic reaction which is accompanied by the pearlite transformation does not exceed about 20.degree. C., when rapid cooling is carried out, for example, in a lead bath at a temperature of 520.degree. C., because the heating is reduced due to the large heat transfer feature of the lead bath. But if a hot-rolled wire rod is cooled by a mist blast method down to a temperature at which the transformation begins and the wire rod is put into an electric heating furnace, the recuperation reaches 70.degree. C., but the pearlite structure of the wire rod obtained therefrom is coarse and the wire rod is far from having mechanical properties equivalent to that of a wire rod produced by the lead patenting. To produce a wire rod of such mechanical properties, it is required to increase the cooling speed and, at the same time, to have the transformation starting temperature go down. But the drop of the starting temperature delays remarkably the completion of the transformation and prolongs the time necessary for maintaining the constant temperature, and therefore, this is disadvantageous in view from equipment and facilities. In addition, in case that the temperature goes excessively down, there is a danger of producing an over cooling structure such as bainite in the products.
Moreover, the increase in the cooling speed for a large diameter wire rod with 13 mm or so gets difficult, compared to cooling a small diameter wire rod with 5.5 mm.
When the diameter becomes large and the starting temperature of the cooling also becomes high, it is required to compulsorily restrain the elevation of the temperature due to the repecuration.